When Tommy and Tuppence Beresford purchase an aging English manor, they uncover a cryptic message hidden within a collection of antique books that suggests a decades-old murder was never solved. As the couple settles into their new home, Tuppence discovers a series of underlinings in a copy of The Black Arrow that spell out a warning regarding a woman named Mary Jordan. The Beresfords must navigate the local village history and the secrets of the house's former inhabitants to determine if the threat remains active. The narrative follows the duo as they apply their investigative experience to a cold case that suddenly feels dangerously current.

This novel represents the final work written by Agatha Christie, and it is often noted for its departure from the tighter, more clinical plotting of her earlier career. The narrative reflects a more reflective, atmospheric tone that prioritizes the charm of the Beresfords over the rapid-fire deduction found in her Poirot or Marple series. Readers often observe that the mystery itself is secondary to the experience of spending time with the aging protagonists in their final literary outing. The book is frequently discussed by enthusiasts as a curiosity that captures the author's late-career style and her enduring interest in the secrets hidden within quiet, domestic spaces.